The present invention relates to a method for manufacturing a protective wafer including a frame wafer and an optical window, and to a method for manufacturing a micromechanical device including such a protective wafer having an inclined optical window.
MEMS components must be protected against harmful outside environmental conditions, for example the penetration of particles and the like. A protection against mechanical contact or destruction and for enabling the separation from a wafer assembly into individual chips by sawing is also necessary. In many cases, the setting of a certain atmosphere (gas type and gas pressure; vacuum also) must be made possible with the aid of a hermetic encapsulation.
The encapsulation of MEMS components with the aid of a cap wafer, which has cavities and through-holes, in the wafer assembly is an established process. For this purpose, a cap wafer is aligned with the wafer including the MEMS structures and joined thereto. The joining may take place both with the aid of anodic bonding and direct bonding (joining agent-free joint between glass and silicon), via eutectic joining layers and via glass solders or adhesives. The MEMS component is usually situated under the cavities of the cap wafer, and the electrical bond pads for connecting the component to thin wires are accessible via the through-hole in the cap wafer.
For optical MEMS (MOEMS), such as for micromirrors, the above-described protection and additionally a transparent window having a high optical quality and, if necessary, also including special optical coatings, are necessary. In isolated instances, through-holes for the electrical connection are also implemented in the caps.
When optical beams pass through the transparent window, reflections are created at the interfaces. When the stationary reflections are in the scan range of the μmirror, their intensity exceeds that of the projected image and thus has an interfering effect. These interfering reflections may only be reduced in their intensity by an anti-reflection coating of the optical window. Since the mirror in general pivots or is deflected symmetrically around its rest position, the reflection is always in the scan range when the optical window is in parallel to the rest position of the mirror surface and the distance between the mirror plane and the optical window is small. This is frequently the case with MEMS. The only option to avoid the interference by the reflections is to guide them out of the scan range in that the optical window and the mirror surface (in rest position, i.e., in the non-deflected state) are not situated in parallel to one another. Two options exist for this: inclination of the optical window or inclination of the rest position of the mirror with respect to a main plane of the MEMS component. Both options are already known from publications and from patent specifications in various specific embodiments. Inclined windows for separated chips are known from EP 1688776 A1, for example.
Inclined windows or also other window shapes with which the reflections are avoidable are described for wafer level packaging in U.S. Patent Application Publication No. 2007/0024549. The three-dimensional surface structures (e.g., inclined windows) described in the latter patent specification are to be manufactured from a transparent material (glass or plastic material) in a wafer assembly. The methods with the aid of which the three-dimensional structures may be manufactured are either very expensive or do not result in the necessary optical quality. Wafers including corresponding three-dimensional structures are moreover problematic during processing (e.g., during wafer bonding) since the structures may be damaged in the process.
Other methods for manufacturing protective caps having inclined optical windows are described, for example, in the German Application Nos. DE 102010062118 A1 and DE 102012206858 A1. However, these methods are also still relatively expensive for high volume applications, or the optical windows have only a limited optical quality.